Is Los Angeles a desert? Our city gets about 15 inches of rainfall annually, slightly more precipitation than, say, Missoula, Montana (though we have fewer days per year that are considered “wet”). Did you know this is enough to serve a fairly large population and irrigate its urban greenery?

But every time it rains an inch in the city of L.A., 3.8 billion gallons of runoff are sent to sea, sweeping trash, toxins, and bacteria into waterways and polluting our beaches and ocean. And what about runoff that doesn’t result from rainfall, but rather from our own wasteful habits of using more than we need for irrigation, washing our cars, and simply running the tap? How can we capture this and put it to responsible use?

In 2008 TreePeople formally opened the Center for Community Forestry at its Coldwater Canyon Park hilltop headquarters. A central feature is an underground rain-harvesting cistern. This 216,000 gallon cistern was designed to demonstrate how—all over L.A.—we can harvest local rainfall and rooftop runoff. Not only does this prevent pollution, but it helps to free the city from reliance on our distant sources of water. During the dry season, filtered water from the cistern irrigates the plants and trees in a 5-acre area of the park.

Jim Hardie, TreePeople’s Director of Park Operations, remembers, “During the first three rainy seasons of full operation of the cistern irrigation system, we filled the cistern completely by either late February or in March. And that wonderful free water got us into late September/early October in each of those years, in terms of irrigation for the hilltop trees and plants—almost perfectly matching our needs until the rains came again.”

Hardie explains that one inch of rainfall contributes approximately 13,500 gallons to the cistern. Last week’s showers helped shore up reserves, but there’s been a long dry spell since December. “The season total for this hilltop so far is something just under 7 inches,” says Hardie, “about half of ‘normal.’”

With 2012 designated worldwide the “hottest year on record,” and indications that things are only heating up, the hot, dry weather both last season and this one has meant that Treepeople has had to supplement cistern-stored water with what we regard as “imported” city water.

“The good news,” says Hardie, “is that the plants and trees on the hilltop have been in the ground for a full five years and are mostly established—so we don’t need to use so much city water to keep them going.”

Check out TreePeople’s rainwater harvesting tips to reduce waste while increasing your access to locally harvested water.